The invention relates to metal/air batteries, and particularly such batteries having gas bubbled through the electrolyte.
Metal/air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell, in which (during cell operation) oxygen dissociates while metal of the anode oxidizes, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) containing a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air batteries using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. The discharge reaction for this cell may be written EQU 4Al+3 0.sub.2 +6H.sub.2 0.fwdarw.4Al(OH).sub.3.
As the reaction proceeds, large amounts of the aluminum hydroxide reaction product forms in the space between anode and cathode, and this ultimately interferes with cell operation, necessitating periodic cleaning and electrolyte replacement. It will be appreciated that cleaning and electrolyte replacement become quite complicated when the battery has multiple cells.
In metal-air batteries of this type, it has been found to be very beneficial to bubble air or other gas into the electrolyte in the gap between the anode and cathode. This is done by placing a bubble tube in the reservoir below the gap between the anode and cathode. The lifting action created by the rising gas in the electrolyte is used to circulate the electrolyte within each individual cell. The bubbling action also has other benefits, such as hydrogen gas dilution and heat removal.
However, there have been difficulties in achieving uniform distribution of the injected gas across a multi-hole bubble tube extending across beneath a cell and the problem is multiplied with a series of bubble tubes in a multi-cell battery. This lack of uniform distribution can be caused by many factors including surface tension effects, variations in air pressure, or plugged injector holes. The result is a reduction in cell performance and/or premature failure. This is especially serious in a multi-cell series battery where failure of one cell can shut down the entire battery.
It is an object of this invention to provide an air system which avoids the above problems.